Dynamic stabilization device including overhanging stabilizing member

ABSTRACT

Spine stabilization devices, systems and methods are provided in which a single resilient member or spring is disposed on an elongate element that spans two attachment members attached to different spinal vertebrae. The elongate element passes through at least one of the two attachment members, permitting relative motion therebetween, and terminates in a stop or abutment. A second resilient member is disposed on the elongate element on an opposite side of the sliding attachment member, e.g., in an overhanging orientation. The two resilient members are capable of applying mutually opposing urging forces, and a compressive preload can be applied to one or both of the resilient members.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of a provisional patentapplication entitled “Dynamic Spine Stabilizer,” filed on Jun. 23, 2004and assigned Ser. No. 60/581,716. The entire contents of the foregoingprovisional patent application are incorporated by reference herein.

BACKGROUND OF THE DISCLOSURE

1. Technical Field

The present disclosure is directed to a dynamic stabilization device andsystem for spinal implantation and, more particularly, to a dynamicstabilization device and system that is adapted to be positioned/mountedrelative to first and second laterally-spaced pedicle screws and thatincludes at least one dynamic stabilization member that is positionedbeyond the region defined between the pedicle screws, e.g., in an“overhanging” orientation.

2. Background Art

Low back pain is one of the most expensive diseases afflictingindustrialized societies. With the exception of the common cold, itaccounts for more doctor visits than any other ailment. The spectrum oflow back pain is wide, ranging from periods of intense disabling painwhich resolve, to varying degrees of chronic pain. The conservativetreatments available for lower back pain include: cold packs, physicaltherapy, narcotics, steroids and chiropractic maneuvers. Once a patienthas exhausted all conservative therapy, the surgical options range frommicro discectomy, a relatively minor procedure to relieve pressure onthe nerve root and spinal cord, to fusion, which takes away spinalmotion at the level of pain.

Each year, over 200,000 patients undergo lumbar fusion surgery in theUnited States. While fusion is effective about seventy percent of thetime, there are consequences even to these successful procedures,including a reduced range of motion and an increased load transfer toadjacent levels of the spine, which accelerates degeneration at thoselevels. Further, a significant number of back-pain patients, estimatedto exceed seven million in the U.S., simply endure chronic low-backpain, rather than risk procedures that may not be appropriate oreffective in alleviating their symptoms.

New treatment modalities, collectively called motion preservationdevices, are currently being developed to address these limitations.Some promising therapies are in the form of nucleus, disc or facetreplacements. Other motion preservation devices provide dynamic internalstabilization of the injured and/or degenerated spine, without removingany spinal tissues. A major goal of this concept is the stabilization ofthe spine to prevent pain while preserving near normal spinal function.The primary difference in the two types of motion preservation devicesis that replacement devices are utilized with the goal of replacingdegenerated anatomical structures which facilitates motion while dynamicinternal stabilization devices are utilized with the goal of stabilizingand controlling abnormal spinal motion.

Over ten years ago a hypothesis of low back pain was presented in whichthe spinal system was conceptualized as consisting of the spinal column(vertebrae, discs and ligaments), the muscles surrounding the spinalcolumn, and a neuromuscular control unit which helps stabilize the spineduring various activities of daily living. Panjabi M M. “The stabilizingsystem of the spine. Part I. Function, dysfunction, adaptation, andenhancement.” J Spinal Disord 5 (4): 383-389, 1992a. A corollary of thishypothesis was that strong spinal muscles are needed when a spine isinjured or degenerated. This was especially true while standing inneutral posture. Panjabi M M. “The stabilizing system of the spine. PartII. Neutral zone and instability hypothesis.” J Spinal Disord 5 (4):390-397, 1992b. In other words, a low-back patient needs to havesufficient well-coordinated muscle forces, strengthening and trainingthe muscles where necessary, so they provide maximum protection whilestanding in neutral posture.

Dynamic stabilization (non-fusion) devices need certain functionality inorder to assist the compromised (injured or degenerated with diminishedmechanical integrity) spine of a back patient. Specifically, the devicesmust provide mechanical assistance to the compromised spine, especiallyin the neutral zone where it is needed most. The “neutral zone” refersto a region of low spinal stiffness or the toe-region of theMoment-Rotation curve of the spinal segment (see FIG. 1). Panjabi M M,Goel V K, Takata K. 1981 Volvo Award in Biomechanics. “PhysiologicalStrains in Lumbar Spinal Ligaments, an in vitro Biomechanical Study.”Spine 7 (3): 192-203, 1982. The neutral zone is commonly defined as thecentral part of the range of motion around the neutral posture where thesoft tissues of the spine and the facet joints provide least resistanceto spinal motion. This concept is nicely visualized on aload-displacement or moment-rotation curve of an intact and injuredspine as shown in FIG. 1. Notice that the curves are non-linear; thatis, the spine mechanical properties change with the amount ofangulations and/or rotation. If we consider curves on the positive andnegative sides to represent spinal behavior in flexion and extensionrespectively, then the slope of the curve at each point representsspinal stiffness. As seen in FIG. 1, the neutral zone is the lowstiffness region of the range of motion.

Experiments have shown that after an injury of the spinal column or dueto degeneration, neutral zones, as well as ranges of motion, increase(see FIG. 1). However, the neutral zone increases to a greater extentthan does the range of motion, when described as a percentage of thecorresponding intact values. This implies that the neutral zone is abetter measure of spinal injury and instability than the range ofmotion. Clinical studies have also found that the range of motionincrease does not correlate well with low back pain. Therefore, theunstable spine needs to be stabilized especially in the neutral zone.Dynamic internal stabilization devices must be flexible so as to movewith the spine, thus allowing the disc, the facet joints, and theligaments normal physiological motion and loads necessary formaintaining their nutritional well-being. The devices must alsoaccommodate the different physical characteristics of individualpatients and anatomies to achieve a desired posture for each individualpatient.

With the foregoing in mind, those skilled in the art will understandthat a need exists for a spinal stabilization device which overcomes theshortcoming of prior art devices. The present invention provides such anapparatus and method for spinal stabilization.

SUMMARY OF THE DISCLOSURE

The present disclosure provides advantageous apparatus and methods forstabilizing adjacent spinal vertebrae in spinal axial rotation andspinal lateral bending. The disclosed stabilization devices and systemsare adapted to be disposed between laterally-spaced pedicle screwsattached to the same spinal vertebra. The disclosed spinal stabilizationdevices/systems are advantageously adapted to include at least first andsecond stabilizing elements which function, according to exemplaryembodiments, in concert for stabilization in spinal flexion and spinalextension. Thus, according to exemplary embodiments of the presentdisclosure, the spinal stabilization devices/systems provide stabilizingfunctionality with laterally-spaced pedicle screws, but in a manner thatis not confined within the region defined between such laterally-spacedpedicle screws. Such spinal stabilization designs offer several clinicaladvantages, including a reduced spatial requirement between thelaterally-spaced pedicle screws since a portion of the stabilizationfunctionality is achieved through structures positioned beyond suchlaterally-spaced region, e.g., in an overhanging orientation relative toone of the laterally-spaced pedicle screws.

According to an exemplary embodiment of the present disclosure, adynamic spine stabilization device/system is provided that is adapted tospan adjacent spinal vertebrae. Attachment members are provided tomount/position the dynamic spine stabilization device/system withrespect to laterally-spaced pedicle screws that are mounted into theadjacent spinal vertebrae. The attachment members of the spinestabilization device/system are generally coupled/mounted with respectto the laterally-spaced pedicle screws and are adapted to couple to thedisclosed spinal stabilization device/system. Of note, the spinalstabilization device/system includes a dynamic element that ispositioned between the first and second pedicle screws, and at least oneadditional dynamic element that is positioned beyond or external to theregion defined by the laterally-spaced pedicle screws.

According to an exemplary embodiment of the present disclosure, firstand second dynamic elements are associated with the disclosed dynamicstabilization device/system. A first dynamic element is positioned on afirst side of an attachment member and a second dynamic element ispositioned on the opposite side of such attachment member. Relativemotion between the pedicle screws, which is based upon and responsive tospinal motion (i.e., in flexion or extension), is stabilized through thecombined contributions of the first and second dynamic elements. Eachdynamic element includes one or more components that contribute to thedynamic response thereof, e.g., one or more springs. According toexemplary embodiments, a pair of springs are associated with each of thedynamic elements, e.g., in a nested configuration. In a furtherexemplary embodiment, each of the dynamic elements includes a singlespring, and the single springs are adapted to operate in concert toprovide an advantageous stabilizing response to spinal motion.

According to other exemplary embodiments of the present disclosure, adynamic stabilization device/system is provided in which an elongaterod/pin extends from both sides of an attachment member. The dynamicstabilization device/system may be equipped with one or more stopsassociated with the elongate rod/pin. First and second resilient membersmay be disposed on the pin (e.g., springs), the first resilient memberbeing located in the region defined between the first attachment memberand the second attachment member, and the second resilient member beinglocated between either the first or the second attachment member and thestop. According to further exemplary embodiments of the presentdisclosure, a compressive preload may be established with respect to afirst resilient member, a second resilient member, or both, to provide adesired stabilizing force profile, as described in greater detailherein.

Exemplary methods of use of the disclosed dynamic stabilization devicesand systems are also provided in accordance with the present disclosure.The disclosed dynamic stabilization devices, systems and methods of usehave a variety of applications and implementations, as will be readilyapparent from the disclosure provided herein. Additional advantageousfeatures and functionalities associated with the present disclosure willbe apparent from the detailed description which follows, particularlywhen read in conjunction with the figures appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist those of ordinary skill in the art in making and using thedisclosed spinal stabilization device/system, reference is made to theaccompanying figures, wherein:

FIG. 1 is Moment-Rotation curve for a spinal segment (intact andinjured), showing low spinal stiffness within the neutral zone.

FIG. 2 is a schematic representation of a spinal segment in conjunctionwith a Moment-Rotation curve for a spinal segment, showing low spinalstiffness within the neutral zone.

FIG. 3 a is a schematic of a spinal stabilization device in conjunctionwith a Force-Displacement curve, demonstrating the increased resistanceprovided within the central zone according to spinal stabilizationsystems wherein a dynamic element is positioned between laterally-spacedpedicle screws.

FIG. 3 b is a Force-Displacement curve demonstrating the change inprofile achieved through spring replacement.

FIG. 3 c is a dorsal view of the spine with a pair of dynamicstabilization devices secured thereto.

FIG. 3 d is a side view showing the exemplary dynamic stabilizationdevice in tension.

FIG. 3 e is a side view showing the exemplary dynamic stabilizationdevice in compression.

FIG. 4 is a schematic of a dynamic spine stabilization device that isadapted to position dynamic elements between laterally-spaced pediclescrews.

FIG. 5 is a schematic of an alternate dynamic spine stabilization devicethat is adapted to position dynamic elements between laterally-spacedpedicle screws.

FIG. 6 is a Moment-Rotation curve demonstrating the manner in whichdynamic stabilization devices using the principles of the presentdisclosure assist in spinal stabilization.

FIG. 7 a is a free body diagram of a dynamic stabilization device inwhich dynamic elements are positioned between laterally-spaced pediclescrews.

FIG. 7 b is a diagram representing the central zone of a spine and theforces associated therewith for dynamic stabilization according to thepresent disclosure.

FIG. 8 is a perspective view of an exemplary dynamic stabilizationdevice in accordance with the present disclosure.

FIG. 9 is an exploded view of the dynamic stabilization device shown inFIG. 8.

FIG. 10 is a detailed perspective view of the distal end of a firstpedicle screw for use in exemplary implementations of the presentdisclosure; according to exemplary embodiments of the presentdisclosure, the second pedicle screw is identical.

FIG. 11 is a detailed perspective view of a first pedicle screw securedto an exemplary attachment member according to the present disclosure.

FIG. 12 is a perspective view of the exemplary dynamic stabilizationdevice shown in FIG. 8 as seen from the opposite side.

FIG. 13 is a perspective view of a dynamic stabilization device of thetype depicted in FIG. 8 with a transverse torsion bar stabilizingmember.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the disclosed dynamic stabilizationsystem/device are presented herein. It should be understood, however,that the disclosed embodiments are merely exemplary of the presentinvention, which may be embodied in various forms. Therefore, thedetails disclosed herein are not to be interpreted as limiting, butmerely as the basis for teaching one skilled in the art how to makeand/or use the devices and systems of the present disclosure.

With reference to FIGS. 2, 3 a-e and 4, a method and apparatus aredisclosed for spinal stabilization. In accordance with a preferredembodiment of the present disclosure, the spinal stabilization method isachieved by securing an internal dynamic spine stabilization device 10between adjacent vertebrae 12, 14 and providing mechanical assistance inthe form of elastic resistance to the region of the spine to which thedynamic spine stabilization device 10 is attached. The elasticresistance is applied as a function of displacement such that greatermechanical assistance is provided while the spine is in its neutral zoneand lesser mechanical assistance is provided while the spine bendsbeyond its neutral zone. Although the term elastic resistance is usedthroughout the body of the present specification, other forms ofresistance may be employed without departing from the spirit or scope ofthe present disclosure.

As those skilled in the art will certainly appreciate, and as mentionedabove, the “neutral zone” is understood to refer to a region of lowspinal stiffness or the toe-region of the Moment-Rotation curve of thespinal segment (see FIG. 2). That is, the neutral zone may be consideredto refer to a region of laxity around the neutral resting position of aspinal segment where there is minimal resistance to intervertebralmotion. The range of the neutral zone is considered to be of majorsignificance in determining spinal stability. Panjabi, M M. “Thestabilizing system of the spine. Part II. Neutral zone and instabilityhypothesis.” J Spinal Disorders 1992; 5(4): 390-397.

In fact, the inventor has previously described the load displacementcurve associated with spinal stability through the use of a “ball in abowl” analogy. According to this analogy, the shape of the bowlindicates spinal stability. A deeper bowl represents a more stablespine, while a more shallow bowl represents a less stable spine. Theinventor previously hypothesized that for someone without spinal injurythere is a normal neutral zone (that part of the range of motion wherethere is minimal resistance to intervertebral motion) with a normalrange of motion, and in turn, no spinal pain. In this instance, the bowlis not too deep nor too shallow. However, when an injury occurs to ananatomical structure, the neutral zone of the spinal column increasesand the ball moves freely over a larger distance. By this analogy, thebowl would be more shallow and the ball less stable, and consequently,pain results from this enlarged neutral zone.

In general, pedicle screws 16, 18 attach the dynamic spine stabilizationdevice 10 to the vertebrae 12, 14 of the spine using well-tolerated andfamiliar surgical procedures known to those skilled in the art. Inaccordance with a preferred embodiment, and as those skilled in the artwill certainly appreciate, a pair of opposed stabilization devices arecommonly used to balance the loads applied to the spine (see FIG. 3 c).The dynamic spine stabilization device 10 assists the compromised(injured and/or degenerated) spine of a back pain patient, and helpsher/him perform daily activities. The dynamic spine stabilization device10 does so by providing controlled resistance to spinal motionparticularly around neutral posture in the region of neutral zone. Asthe spine bends forward (flexion) the stabilization device 10 istensioned (see FIG. 3 d) and when the spine bends backward (extension)the stabilization device 10 is compressed (see FIG. 3 e).

The resistance to displacement provided by the dynamic spinestabilization device 10 is non-linear, being greatest in its centralzone so as to correspond to the individual's neutral zone; that is, thecentral zone of the stabilization device 10 provides a high level ofmechanical assistance in supporting the spine. As the individual movesbeyond the neutral zone, the increase in resistance decreases to a moremoderate level. As a result, the individual encounters greaterresistance to movement (or greater incremental resistance) while movingwithin the neutral zone.

The central zone of the dynamic spine stabilization device 10, that is,the range of motion in which the spine stabilization device 10 providesthe greatest resistance to movement, may be adjustable at the time ofsurgery to suit the neutral zone of each individual patient. In suchexemplary embodiments, the resistance to movement provided by thedynamic spine stabilization device 10 is adjustable pre-operatively andintra-operatively. This helps to tailor the mechanical properties of thedynamic spine stabilization device 10 to suit the compromised spine ofthe individual patient. The length of the dynamic spine stabilizationdevice 10 may also be adjustable intra-operatively to suit individualpatient anatomy and to achieve desired spinal posture. The dynamic spinestabilization device 10 can be re-adjusted post-operatively with asurgical procedure to adjust its central zone to accommodate a patient'saltered needs.

According to exemplary embodiments of the present disclosure, balljoints 20, 22 link the dynamic spine stabilization device 10 with thepedicle screws 16, 18. The junction of the dynamic spine stabilizationdevice 10 and pedicle screws 16, 18 is free and rotationallyunconstrained. Therefore, first of all, the spine is allowed allphysiological motions of bending and twisting and second, the dynamicspine stabilization device 10 and the pedicle screws 16, 18 areprotected from harmful bending and torsional forces, or moments. Whileball joints are disclosed in accordance with a preferred/exemplaryembodiment of the present disclosure, other linking structures may beutilized without departing from the spirit or scope of the presentdisclosure.

As there are ball joints 20, 22 at each end of the stabilization device10, no bending moments can be transferred from the spine to thestabilization device 10. Further, it is important to recognize the onlyforces that act on the stabilization device 10 are those due to theforces of the springs 30, 32 within it. These forces are solelydependent upon the tension and compression of the stabilizer 10 asdetermined by the spinal motion. In summary, the stabilization device 10sees only the spring forces. Irrespective of the large loads on thespine, such as when a person carries or lifts a heavy load, the loadscoming to the stabilization device 10 are only the forces developedwithin the stabilization device 10, which are the result of spinalmotion and not the result of the spinal load. The stabilization device10 is, therefore, uniquely able to assist the spine without enduring thehigh loads of the spine, allowing a wide range of design options.

The loading of the pedicle screws 16, 18 in the present stabilizationdevice 10 is also quite different from that in prior art pedicle screwfixation devices. The only load the stabilizer pedicle screws 16, 18 seeis the force from the stabilization device 10. This translates into pureaxial force at the ball joint-screw interface. This mechanism greatlyreduces the bending moment placed onto the pedicle screws 16, 18 ascompared to prior art pedicle screw fusion systems. Due to the balljoints 20, 22, the bending moment within the pedicle screws 16, 18 isessentially zero at the ball joints 20, 22 and it increases toward thetip of the pedicle screws 16, 18. The area of pedicle screw-boneinterface which often is the failure site in a typical prior art pediclescrew fixation device, is the least stressed site, and is therefore notlikely to fail. In sum, the pedicle screws 16, 18, when used inconjunction with the present invention, carry significantly less loadand are placed under significantly less stress than typical pediclescrews.

In FIG. 2, the Moment-Rotation curve for a healthy spine is shown inconfigurations with stabilization device 10. This curve shows the lowresistance to movement encountered in the neutral zone of a healthyspine. However, when the spine is injured, this curve changes and thespine becomes unstable, as evidenced by the expansion of the neutralzone (see FIG. 1).

In accordance with a preferred embodiment of the present invention,people suffering from spinal injuries are best treated through theapplication of increased mechanical assistance in the neutral zone. Asthe spine moves beyond the neutral zone, the necessary mechanicalassistance decreases and becomes more moderate. In particular, and withreference to FIG. 3 a, the support profile contemplated in accordancewith the present invention is disclosed.

Three different profiles are shown in FIG. 3 a. The disclosed profilesare merely exemplary and demonstrate the possible support requirementswithin the neutral zone. Profile 1 is exemplary of an individualrequiring great assistance in the neutral zone and the central zone ofthe stabilizer is therefore increased providing a high level ofresistance over a great displacement; Profile 2 is exemplary of anindividual where less assistance is required in the neutral zone and thecentral zone of the stabilizer is therefore more moderate providingincreased resistance over a more limited range of displacement; andProfile 3 is exemplary of situations where only slightly greaterassistance is required in the neutral zone and the central zone of thestabilizer may therefore be decreased to provide increased resistanceover even a smaller range of displacement.

As those skilled in the art will certainly appreciate, the mechanicalassistance required and the range of the neutral zone will vary fromindividual to individual. However, the basic tenet of the presentinvention remains; that is, greater mechanical assistance for thoseindividuals suffering from spinal instability is required within theindividual's neutral zone. This assistance is provided in the form ofgreater resistance to movement provided within the neutral zone of theindividual and the central zone of the dynamic spine stabilizer 10.

The dynamic spine stabilization device 10 provides mechanical assistancein accordance with the disclosed support profile. Further, thestabilization device 10 may advantageously provide for adjustability viaa concentric spring design.

More specifically, the dynamic spine stabilization device 10 providesassistance to the compromised spine in the form of increased resistanceto movement (provided by springs in accordance with a preferredembodiment) as the spine moves from the neutral posture, in anyphysiological direction. As mentioned above, the Force-Displacementrelationship provided by the dynamic spine stabilization device 10 isnon-linear, with greater incremental resistance around the neutral zoneof the spine and central zone of the stabilization device 10, anddecreasing incremental resistance beyond the central zone of the dynamicspine stabilization device 10 as the individual moves beyond the neutralzone (see FIG. 3 a).

The relationship of stabilization device 10 to forces applied duringtension and compression is further shown with reference to FIG. 3 a. Asdiscussed above, the behavior of the stabilization device 10 isnon-linear. The Load-Displacement curve has three zones: tension,central and compression. If K1 and K2 define the stiffness values in thetension and compression zones respectively, the present stabilizer isdesigned such that the high stiffness in the central zone is “K1+K2”.Depending upon the preload of the stabilization device 10 as will bediscussed below in greater detail, the width of the central zone and,therefore, the region of high stiffness can be adjusted.

With reference to FIG. 4, a dynamic spine stabilization device 10 inaccordance with one aspect of the present disclosure is schematicallydepicted. The dynamic spine stabilization device 10 includes a supportassembly in the form of a housing 20 composed of a first housing member22 and a second housing member 24. The first housing member 22 and thesecond housing member 24 are telescopically connected via externalthreads formed upon the open end 26 of the first housing member 22 andinternal threads formed upon the open end 28 of the second housingmember 24. In this way, the housing 20 is completed by screwing thefirst housing member 22 into the second housing member 24. As such, andas will be discussed below in greater detail, the relative distancebetween the first housing member 22 and the second housing member 24 canbe readily adjusted for the purpose of adjusting the compression of thefirst spring 30 and second spring 32 contained within the housing 20.Although springs are employed in accordance with a preferred embodimentof the present disclosure, other elastic members may be employed withoutdeparting from the spirit or scope of the present disclosure. A pistonassembly 34 links the first spring 30 and the second spring 32 to firstand second ball joints 36, 38. The first and second ball joints 36, 38are in turn shaped and designed for selective attachment to pediclescrews 16, 18 extending from the respective vertebrae 12, 14.

The first ball joint 36 is secured to the closed end 38 of the firsthousing member 20 via a threaded engagement member 40 shaped anddimensioned for coupling, with threads formed within an aperture 42formed in the closed end 38 of the first housing member 22. In this way,the first ball joint 36 substantially closes off the closed end 38 ofthe first housing member 22. The length of the dynamic spinestabilization device 10 may be readily adjusted by rotating the firstball joint 36 to adjust the extent of overlap between the first housingmember 22 and the engagement member 40 of the first ball joint 36. Asthose skilled in the art will certainly appreciate, a threadedengagement between the first housing member 22 and the engagement member40 of the first ball joint 36 is disclosed in accordance with apreferred embodiment, although other coupling structures may be employedwithout departing from the spirit or scope of the present disclosure.

The closed end 44 of the second housing member 24 is provided with a cap46 having an aperture 48 formed therein. As will be discussed below ingreater detail, the aperture 48 is shaped and dimensioned for thepassage of a piston rod 50 from the piston assembly 34 therethrough.

The piston assembly 34 includes a piston rod 50; first and secondsprings 30, 32; and retaining rods 52. The piston rod 50 includes a stopnut 54 and an enlarged head 56 at its first end 58. The enlarged head 56is rigidly connected to the piston rod 50 and includes guide holes 60through which the retaining rods 52 extend during operation of dynamicspine stabilization device 10. As such, the enlarged head 56 is guidedalong the retaining rods 52 while the second ball joint 38 is movedtoward and away from the first ball joint 36. As will be discussed belowin greater detail, the enlarged head 56 interacts with the first spring30 to create resistance as the dynamic spine stabilization device 10 isextended and the spine is moved in flexion.

A stop nut 54 is fit over the piston rod 50 for free movement relativethereto. However, movement of the stop nut 54 toward the first balljoint 36 is prevented by the retaining rods 52 that support the stop nut54 and prevent the stop nut 54 from moving toward the first ball joint36. As will be discussed below in greater detail, the stop nut 54interacts with the second spring 32 to create resistance as the dynamicspine stabilizer 10 is compressed and the spine is moved in extension.

The second end 62 of the piston rod 50 extends from the aperture 48 atthe closed end 44 of the second housing member 24, and is attached to anengagement member 64 of the second ball joint 38. The second end 62 ofthe piston rod 50 is coupled to the engagement member 64 of the secondball joint 38 via a threaded engagement. As those skilled in the artwill certainly appreciate, a threaded engagement between the second end62 of the piston rod 50 and the engagement member 64 of the second balljoint 38 is disclosed in accordance with a preferred embodiment,although other coupling structures may be employed without departingfrom the spirit of the present invention.

As briefly mentioned above, the first and second springs 30, 32 are heldwithin the housing 20. In particular, the first spring 30 extendsbetween the enlarged head 56 of the piston rod 50 and the cap 46 of thesecond housing member 24. The second spring 32 extends between thedistal end of the engagement member 64 of the second ball joint 38 andthe stop nut 54 of the piston rod 50. The preloaded force applied by thefirst and second springs 30, 32 holds the piston rod in a staticposition within the housing 20, such that the piston rod is able to moveduring either extension or flexion of the spine.

In use, when the vertebrae 12, 14 are moved in flexion and the firstball joint 36 is drawn away from the second ball joint 38, the pistonrod 50 is pulled within the housing 24 against the force being appliedby the first spring 30. In particular, the enlarged head 56 of thepiston rod 50 is moved toward the closed end 44 of the second housingmember 24. This movement causes compression of the first spring 30,creating resistance to the movement of the spine. With regard to thesecond spring 32, the second spring 32 moves with the piston rod 50 awayfrom second ball joint 38. As the vertebrae move in flexion within theneutral zone, the height of the second spring 32 is increased, reducingthe distractive force, and in effect increasing the resistance of thedevice to movement. Through this mechanism, as the spine moves inflexion from the initial position both spring 30 and spring 32 resistthe distraction of the device directly, either by increasing the loadwithin the spring (i.e. first spring 30) or by decreasing the loadassisting the motion (i.e. second spring 32).

However, when the spine is in extension, and the second ball joint 38 ismoved toward the first ball joint 36, the engagement member 64 of thesecond ball joint 38 moves toward the stop nut 54, which is held isplace by the retaining rods 52 as the piston rod 50 moves toward thefirst ball joint 36. This movement causes compression of the secondspring 32 held between the engagement member 64 of the second ball joint38 and the stop nut 54, to create resistance to the movement of thedynamic spine stabilization device 10. With regard to the first spring30, the first spring 30 is supported between the cap 46 and the enlargedhead 56, and as the vertebrae move in extension within the neutral zone,the height of the second spring 30 is increased, reducing thecompressive force, and in effect increasing the resistance of the deviceto movement. Through this mechanism, as the spine moves in extensionfrom the initial position both spring 32 and spring 30 resist thecompression of the device directly, either by increasing the load withinthe spring (i.e. second spring 32) or by decreasing the load assistingthe motion (i.e. first spring 30).

Based upon the use of two concentrically positioned elastic springs 30,32 as disclosed in accordance with the present disclosure, an assistance(force) profile as shown in FIG. 2 is provided by the present dynamicspine stabilizer 10. That is, the first and second springs 30, 32 workin conjunction to provide a large elastic force when the dynamic spinestabilization device 10 is displaced within the central zone. However,once displacement between the first ball joint 36 and the second balljoint 38 extends beyond the central zone of the stabilization device 10and the neutral zone of the individual's spinal movement, theincremental resistance to motion is substantially reduced as theindividual no longer requires the substantial assistance needed withinthe neutral zone. This is accomplished by setting the central zone ofthe device disclosed herein. The central zone of the force displacementcurve is the area of the curve which represents when both springs areacting in the device as described above. When the motion of the spine isoutside the neutral zone and the correlating device elongation orcompression is outside the set central zone, the spring which iselongating reaches its free length. Free length, as anybody skilled inthe art will appreciate, is the length of a spring when no force isapplied. In this mechanism the resistance to movement of the deviceoutside the central zone (where both springs are acting to resistmotion) is only reliant on the resistance of one spring: either spring30 in flexion or spring 32 in extension.

As briefly discussed above, dynamic spine stabilization device 10 may beadjusted by rotation of the first housing member 22 relative to thesecond housing member 24. This movement changes the distance between thefirst housing member 22 and the second housing member 24 in a mannerwhich ultimately changes the preload placed across the first and secondsprings 30, 32. This change in preload alters the resistance profile ofthe present dynamic spine stabilization device 10 from that shown inProfile 2 of FIG. 3 a to an increase in preload (see Profile 1 of FIG. 3a) which enlarges the effective range in which the first and secondsprings 30, 32 act in unison. This increased width of the central zoneof the stabilization device 10 correlates to higher stiffness over alarger range of motion of the spine. This effect can be reversed asevident in Profile 3 of FIG. 3 a.

The dynamic spine stabilization device 10 is attached to pedicle screws16, 18 extending from the vertebral section requiring support. Duringsurgical attachment of the dynamic spine stabilization device 10, themagnitude of the stabilizer's central zone can be adjusted for eachindividual patient, as judged by the surgeon and/or quantified by aninstability measurement device. This optional adjustable feature ofdynamic spine stabilization device 10 is exemplified in the threeexplanatory profiles that have been generated in accordance with thepresent disclosure (see FIG. 2; note the width of the device centralzones).

Pre-operatively, the first and second elastic springs 30, 32 of thedynamic spine stabilization device 10 can be replaced by a different setto accommodate a wider range of spinal instabilities. As expressed inFIG. 3 b, Profile 2 b demonstrates the force displacement curvegenerated with a stiffer set of springs when compared with the curveshown in Profile 2 a of FIG. 3 b.

Intra-operatively, the length of the dynamic spine stabilization device10 is adjustable by turning the engagement member 40 of the first balljoint 36 to lengthen the stabilization device 10 in order to accommodatedifferent patient anatomies and desired spinal posture. Pre-operatively,the piston rod 50 may be replaced to accommodate an even wider range ofanatomic variation.

The dynamic spine stabilization device 10 has been tested alone for itsload-displacement relationship. When applying tension, the dynamic spinestabilization device 10 demonstrated increasing resistance up to apre-defined displacement, followed by a reduced rate of increasingresistance until the device reached its fully elongated position. Whensubjected to compression, the dynamic spine stabilization device 10demonstrated increasing resistance up to a pre-defined displacement,followed by a reduced rate of increasing resistance until the devicereached its fully compressed position. Therefore, the dynamic spinestabilization device 10 exhibits a load-displacement curve that isnon-linear with the greatest resistance to displacement offered aroundthe neutral posture. This behavior helps to normalize theload-displacement curve of a compromised spine.

In another embodiment of an aspect of the disclosed design and withreference to FIG. 5, the stabilization device 110 may be constructedwith an in-line spring arrangement. In accordance with this embodiment,the housing 120 is composed of first and second housing members 122, 124which are coupled with threads allowing for adjustability. A first balljoint 136 extends from the first housing member 122. The second housingmember 124 is provided with an aperture 148 through which the second end162 of piston rod 150 extends. The second end 162 of the piston rod 150is attached to the second ball joint 138. The second ball joint 138 isscrewed onto the piston rod 150.

The piston rod 150 includes an enlarged head 156 at its first end 158.The first and second springs 130, 132 are respectively secured betweenthe enlarged head 156 and the closed ends 138, 144 of the first andsecond housing members 122, 124. In this way, the stabilization device110 provides resistance to both expansion and compression using the samemechanical principles described for the previous embodiment.

Adjustment of the resistance profile in accordance with this alternateembodiment is achieved by rotating the first housing member 122 relativeto the second housing member 124. Rotation in this way alters thecentral zone of high resistance provided by the stabilization device110. As previously described one or both springs may also be exchangedto change the slope of the force-displacement curve in two or threezones respectively.

To explain how the stabilization device 10, 110 assists a compromisedspine (increased neutral zone), reference is made to the moment-rotationcurves (FIG. 6). Four curves are shown: 1. Intact, 2. Injured, 3.Stabilizer and, 4. Injured+Stabilizer. These are, respectively, theMoment-Rotation curves of the intact spine, injured spine, stabilizeralone, and stabilizer plus injured spine Notice that this curve is closeto the intact curve. Thus, the stabilization device, which providesgreater resistance to movement around the neutral posture, is ideallysuited to compensate for the instability of the spine.

With reference to FIGS. 8 to 13, a stabilization device 210 according tothe present disclosure is schematically depicted. This embodimentpositions the first and second springs 230, 232 on opposite sides of apedicle screw 218. As with the earlier embodiments, the stabilizationdevice 210 includes a housing 220 having a first attachment member 260with a first ball joint 262 extending from a first end 264 of thehousing 220 and a second attachment member 266 with second ball joint268 extending through a central portion of the stabilizer 220. Each ofthe ball joints 262, 268 is composed of a socket 270 a, 270 b with aball 272 a, 272 b secured therein.

More particularly, each of the pedicle screws 216, 218 includes aproximal end 274 and a distal end 276 (as the first and second pediclescrews 216, 218 are identical, similar numerals will be used indescribing them). The proximal end 274 includes traditional threading278 adapted for secure attachment along the spinal column of anindividual. The distal end 276 of the pedicle screw 216, 218 is providedwith a collet 278 adapted for engagement within a receiving aperture 280a, 280 b formed within the ball 272 a, 272 b of the first and secondattachment members 260, 266 of the stabilization device 210.

The collet 278 at the distal end 276 of the pedicle screw 216, 218 isformed with the ability to expand and contract under the control of themedical practitioner installing the present stabilizer 210. The collet278 is composed of a plurality of flexible segments 282 with a centralaperture 284 therebetween. As will be explained below in greater detail,the flexible segments 282 are adapted for movement between an expandedstate used to lock the collet 278 within the receiving aperture 280 a,280 b of the ball 272 a, 272 b and an unexpanded state wherein thecollet 278 may be selectively inserted or removed from the receivingaperture 280 a, 280 b of the ball 272 a, 272 b.

The receiving apertures 280 a, 280 b of the respective balls 272 a, 272b are shaped and dimensioned for receiving the collet 278 of the pediclescrew 216, 218 while it is in its unexpanded state. Retention of thecollet 278 is further enhanced by the provision of a lip 286 at thedistal end 276 of the collet 278. The lip 286 is shaped and dimensionedto grip the receiving aperture 280 a, 280 b for retaining the collet 278therein.

Expansion of the collet 278 of pedicle screw 216, 218 is achieved by theinsertion of a set screw 288 within the central aperture 284 formedbetween the various segments 282 of the pedicle screw collet 278. As theset screw 288 is positioned within the central aperture 284, thesegments 282 are forced outwardly. This increases the effective diameterof the collet 278 and ultimately brings the outer surface of the collet278 into contact with the receiving aperture 280 a, 280 b, securelylocking the collet 278, that is, the distal end 276 of the pedicle screw216, 218 within the receiving aperture 280 a, 280 b of the ball 272 a,272 b.

Access for the insertion of the set screw 288 within the centralaperture 284 of the collet 278 is provided by extending the receivingaperture 280 a, 280 b the entire way through the ball 272 a, 272 b. Inthis way, the collet 278 is placed within the receiving aperture 280 a,280 b of the ball 272 a, 272 b while in its unexpanded state, the setscrew 288 is inserted within the central aperture 284 between thevarious segments 282 to cause the segments 282 to expand outwardly andlock the collet 278 within the receiving aperture 280 a, 280 b. Inaccordance with a preferred embodiment, the set screw 288 is securedwithin the central aperture 284 via mating threads formed along theinner surface along of the central aperture and the outer surface of theset screw 288.

Although the present ball joint/pedicle screw structure has beendisclosed with reference to a particle stabilizer structure, thoseskilled in the art will appreciate that the ball joint/pedicle screwstructure may be employed with various stabilizer structures withoutdeparting from the spirit of the present invention. In fact, it iscontemplated the disclosed connection structure may be employed in avariety of environments without departing from the spirit of the presentinvention.

With reference to the stabilization device 210, an alignment pin 250extends from the first attachment member 260 through a bearing aperture290 within the second attachment member 266. The alignment pin 250includes an abutment member 256 at its free end 258. First and secondsprings 230, 232 are concentrically positioned about the alignment pin250. The first spring 230 is positioned to extend between the firstattachment member 260 and the second attachment member 266, while thesecond spring 232 is positioned to extend between the second attachmentmember 266 and the abutment member 256 at the free end 258 of thealignment pin 250. The arrangement of the alignment pin 250, first andsecond attachment members 260, 266 and first and second springs 230, 232allows for resistive translation of the alignment pin 250 relative tothe vertebrae. In practice, the alignment pin 250, springs 230, 232 andattachment members 260, 266 are arranged to create a compressive preloadacross the system.

This design allows for an axial configuration which generates thedesired Force-Displacement curves as shown with reference to FIG. 3,while allowing for a much shorter distance between the first and secondattachment members. The stabilization device disclosed above may also beused in the stabilization of multiple level systems. It is contemplatedthat stabilization on multiple levels may be achieved through theimplementation of multiple alignment pins coupled via multiple springsets and pedicle screws.

The alignment pin 250 also provides tensile force for achieving thepreload utilized in conjunction with the springs 230, 232. In accordancewith an exemplary embodiment, the alignment pin 250 is flexible andprovides flexible guidance for the springs 230, 232 without debriscausing bearing surfaces, provides tensile for the preload, provides alow friction, straight bearing surface as it moves through the secondattachment member 266 and functions at times as a straight member and atother times as a flexible guide for springs 230, 232.

As mentioned above, the alignment pin 250 is cable of functioning asboth a straight guide member and as a flexible guide member. Thedetermination as to whether the alignment pin 250 functions as astraight guide member or a flexible guide member for the springs 230,232 is generally based upon location of the alignment pin 250 relativeto the remaining stabilization device 210 components as the spine moves.This functionality is especially important during flexion. In accordancewith an exemplary embodiment, the alignment pin 250 has a uniform crosssectional shape capable of performing as both a straight guide memberand a flexed guide member.

In accordance with yet a further embodiment, and with reference to FIG.13, the stabilization device 210 may be used in conjunction with atorsion bar 292 connecting the stabilization device 210 to adjacentstabilizers as shown in FIG. 3 c. In accordance with an exemplaryembodiment, the torsion bar 292 is connected to the attachment members260, 266 of adjacent stabilization devices with conventional connectionstructures. The use of the torsion bar 292 increases stability in axialrotation or lateral bending. The torsion bar 292 generally has a uniformcross section for purposes where uniform torsion is required. However,and in accordance with exemplary embodiments of the present disclosure,it is contemplated that the torsion bar 292 may have an asymmetric crosssection so as to provide for flexibility of stiffness in two planes. Insuch instances, the asymmetric cross sectional torsion bar 292 willaffect the system stiffness in lateral bending and axial rotationindependently. Further, the torsion bar 292 may be utilized to tune thesystems stabilization in all three planes.

In addition to the dynamic spine stabilization device described above,other complementary devices are contemplated. For example, a link-devicemay be provided for joining the left- and right-stabilizer units to helpprovide additional stability in axial rotation and lateral bending. Thislink-device would be a supplement to the dynamic spine stabilizationdevice and would be applied as needed on an individual patient basis. Inaddition, a spinal stability measurement device may be utilized. Themeasurement device would be used to quantify the stability of eachspinal level at the time of surgery. This device would attachintra-operatively to a pair of adjacent spinal components at compromisedand uncompromised spinal levels to measure the stability of each level.The stability measurements of the adjacent uninjured levels relative tothe injured level(s) can be used to determine the appropriate adjustmentof the device. Additionally, the stability measurements of the injuredspinal level(s) can be used to adjust the device by referring to atabulated database of normal uninjured spinal stabilities. The devicewill be simple and robust, so that the surgeon is provided with theinformation in the simplest possible manner under operative conditions.

The choice of spring(s) to be used in accordance with the presentdisclosure to achieve the desired force profile curve is governed by thebasic physical laws governing the force produced by springs. Inparticular, the force profile described above and shown in FIG. 3 a isachieved through the unique design of the present stabilizer.

First, the stabilization device functions both in compression andtension, even through the two springs within the stabilizer are both ofcompression type. Second, the higher stiffness (K₁+K₂) provided by thestabilization device in the central zone is due to the presence of apreload. Both springs are made to work together, when the preload ispresent. As the stabilization device is either tensioned or compressed,the force increases in one spring and decreases in the other. When thedecreasing force reaches the zero value, the spring corresponding tothis force no longer functions, thus decreasing the stabilization devicefunction, an engineering analysis, including the diagrams shown in FIGS.7 a and 7 b, is presented below (the analysis specifically relates tothe embodiment disclosed in FIG. 5, although those skilled in the artwill appreciate the way in which it applies to all embodiments disclosedin accordance with the present invention).

-   -   F₀ is the preload within the stabilization device, introduced by        shortening the body length of the housing as discussed above.    -   K₁ and K₂ are stiffness coefficients of the compression springs,        active during stabilization device tensioning and compression,        respectively.    -   F and D are respectively the force and displacement of the disc        of the stabilization device with respect to the body of the        stabilizer.    -   The sum of forces on the disc must equal zero. Therefore,        F+(F ₀ −D×K ₂)−(F ₀ +D×K ₁)=0, and        F=D×(K ₁ +K ₂).    -   With regard to the central zone (CZ) width (see FIG. 3 a):        -   On Tension side CZ_(T) is:            CZ _(T) =F ₀ /K ₂.        -   On Compression side CZ_(c) is:            CZ _(c) =F ₀ /K ₁.

As those skilled in the art will certainly appreciate, the conceptsunderlying the present disclosure may be applied to other medicalprocedures. As such, these concepts may be utilized beyond spinaltreatments without departing from the spirit or scope of the presentinvention.

While exemplary embodiments have been shown and described, it will beunderstood that there is no intent to limit the invention by suchdisclosure, but rather, is intended to cover all modifications andalternate constructions falling within the spirit and scope of theinvention.

1. A dynamic stabilization device, comprising: an elongate elementhaving an elongate aspect configured and dimensioned for spanning a pairof adjacent spinal vertebrae; first and second stops coupled to theelongate element in spaced relation; a first attachment member sized andshaped so as to permit the first attachment member to be coupled to apedicle screw, the first attachment member adapted to be coupled withrespect to the elongate element between said first and second stops suchthat relative motion between the first attachment member and theelongate member is permitted; a first resilient member disposed withrespect to the elongate element between the first stop and the firstattachment member; and a second resilient member disposed with respectto the elongate element between the first attachment member and thesecond stop; wherein the first and second resilient members cooperate todeliver a non-linear force displacement response to relative movementbetween said first stop and said first attachment member; and whereinthe first and second resilient members comprise metallic coil springsconcentrically disposed around the elongate element.
 2. A dynamicstabilization device according to claim 1, wherein the elongate elementdefines an axis of travel, and wherein the first attachment member isadapted to assume an intermediate position along the axis of travel. 3.A dynamic stabilization device according to claim 1, wherein the firstand second resilient members simultaneously apply first and secondurging forces with respect to the pedicle screw.
 4. A dynamicstabilization device according to claim 3, wherein the first and secondurging forces have opposite orientations.
 5. A dynamic stabilizationdevice according to claim 1, wherein the first and second resilientmembers comprise axially-oriented springs.
 6. A dynamic stabilizationdevice according to claim 1, further comprising a second attachmentmember that is adapted to be coupled with respect to the elongateelement such that the first resilient member is disposed between thefirst attachment member and the second attachment member.
 7. A dynamicstabilization device according to claim 6, wherein the second attachmentmember is adjacent to the first stop.
 8. A dynamic stabilization deviceaccording to claim 6, wherein at least one of said first and secondattachment members includes a joint for coupling to a head of a pediclescrew so as to permit said at least one attachment member to move withrespect to the head of the pedicle screw in all three spatial planes. 9.A dynamic stabilization device according to claim 1, wherein theelongate element defines a travel path between the first and secondstops of a predetermined length.
 10. A dynamic stabilization deviceaccording to claim 1, wherein at least one of the first and secondresilient members is selectively removable from the elongate element andreplaceable with a replacement resilient member having differentstiffness properties.
 11. A dynamic stabilization device according toclaim 1, wherein the first attachment member includes a sleevepermitting the attachment member to be coupled to the elongate elementand to slide relative to the elongate element along a travel path.
 12. Adynamic stabilization device according to claim 1, wherein the elongateelement defines a travel path that is substantially straight.
 13. Adynamic stabilization device according to claim 1, wherein the elongateelement defines a travel path that includes at least one curved segment.14. A dynamic stabilization device according to claim 1, wherein theelongate element is flexible with respect to transverse bending.
 15. Thedynamic stabilization device of claim 1, wherein said non-linear forcedisplacement response is characterized by a first force displacementcurve in proximity to a central zone of the spine, a second displacementcurve associated with tension of the spine removed from said centralzone that differs from said first force displacement curve, and a thirddisplacement curve associated with compression of the spine removed fromsaid central zone that also differs from said first displacement curve.16. A dynamic stabilization device, comprising: an elongate elementhaving an elongate aspect configured and dimensioned for spanning a pairof adjacent spinal vertebrae; first and second stops coupled to theelongate element in spaced relation; a first attachment member sized andshaped so as to permit the first attachment member to be coupled to apedicle screw, the first attachment member adapted to be coupled withrespect to the elongate element between said first and second stops suchthat relative motion between the first attachment member and theelongate member is permitted; a first resilient member disposed withrespect to the elongate element between the first stop and the firstattachment member; a second resilient member disposed with respect tothe elongate element between the first attachment member and the secondstop; and a second attachment member that is adapted to be coupled withrespect to the elongate element such that the first resilient member isdisposed between the first attachment member and the second attachmentmember; wherein the first and second resilient members cooperate todeliver a non-linear force displacement response to relative movementbetween said first stop and said first attachment member; and whereinthe first stop is of unitary construction with respect to the secondattachment member.
 17. A dynamic spine stabilization device, comprising:a pin having an elongate aspect for spanning a pair of adjacent spinalvertebrae; a stop coupled to the pin; a first attachment member sizedand shaped so as to permit the first attachment member to be affixedrelative to one vertebra of the pair of adjacent spinal vertebrae, thefirst attachment member being coupled to the pin in spaced relation tothe stop along a path of extension of the pin; a second attachmentmember sized and shaped so as to permit the second attachment member tobe affixed relative to the other vertebra of the pair of adjacent spinalvertebrae, the second attachment member being movably coupled to the pinsuch that the second attachment member is permitted to translate alongthe path of extension of the pin; a first resilient member disposed onthe pin between the first attachment member and the second attachmentmember and operative, while in a compressed state, to apply a firsturging force to the second attachment member having a longitudinal forcecomponent aligned with the path of extension of the pin and directedaway from the first attachment member; and a second resilient memberdisposed on the pin between the second attachment member and the stopand operative, while in a compressed state, to apply a second urgingforce to the second attachment member having a longitudinal forcecomponent aligned with the path of extension of the pin and directedaway from the stop; wherein the first and second resilient memberscooperate to deliver a non-linear force displacement response torelative movement between said first and second attachment members. 18.A dynamic spine stabilization device according to claim 17, wherein thesecond attachment member is adapted to assume a neutral force positionalong the path of extension of the pin at which the first and secondresilient members simultaneously apply the respective first and secondurging forces, and wherein the respective longitudinal force componentsof the first and second urging forces have opposite orientations.
 19. Adynamic spine stabilization device according to claim 17, wherein thepin is flexible with respect to transverse bending without affecting theurging functions of the first and second resilient members.
 20. Thedynamic stabilization device of claim 17, wherein said non-linear forcedisplacement response is characterized by a first force displacementcurve in proximity to a central zone of the spine, a second displacementcurve associated with tension of the spine removed from said centralzone that differs from said first force displacement curve, and a thirddisplacement curve associated with compression of the spine removed fromsaid central zone that also differs from said first displacement curve.21. A dynamic stabilization device, comprising: an elongate elementdefining a free end; a first resilient member mounted with respect tothe elongate element; a second resilient member mounted with respect tothe elongate element; a first attachment member mounted with respect tothe elongate element ; a second attachment member mounted with respectto the elongate member; and an abutment member mounted with respect tothe free end of the elongate member; wherein the first resilient memberis positioned between the first attachment member and the secondattachment member; wherein the second resilient member is positionedbetween the second attachment member and the abutment member; andwherein the first and second resilient members cooperate to deliver anon-linear force displacement response to relative movement between saidfirst and second attachment members; and wherein the first resilientmember includes at least one spring positioned concentrically relativeto the elongate element.
 22. The dynamic stabilization device of claim21, wherein the elongate element is selected from the group consistingof a straight guide member and a flexible guide member.
 23. The dynamicstabilization device of claim 21, wherein the second attachment memberis mounted with respect to a pedicle screw and the free end of theelongate element is cantilevered relative to the pedicle screw.
 24. Thedynamic stabilization device of claim 21, wherein the second resilientmember includes at least one spring positioned concentrically relativeto the elongate element.
 25. The dynamic stabilization device of claim21, wherein the first attachment member is associated with a first balljoint .
 26. The dynamic stabilization device of claim 25, wherein thefirst ball joint includes a first socket with a first ball mountedwithin the first socket.
 27. The dynamic stabilization device of claim21, wherein the second attachment member is associated with a secondball joint.
 28. The dynamic stabilization device of claim 27, whereinthe second ball joint includes a second socket with a second ballmounted within the second socket.
 29. The dynamic stabilization deviceof claim 21, wherein mounting of the elongate element with respect tothe first attachment member, the second attachment member, the firstresilient member and the second resilient member permits resistivetranslation of the elongate element.
 30. The dynamic stabilizationdevice of claim 21, wherein the elongate element is mounted with respectto the second attachment member so as to allow movement of the elongateelement relative to the second attachment member.
 31. The dynamicstabilization device of claim 21, wherein the abutment member functionsas a stop.
 32. The dynamic stabilization device of claim 21, whereinsaid non-linear force displacement response is characterized by a firstforce displacement curve in proximity to a central zone of the spine, asecond displacement curve associated with tension of the spine removedfrom said central zone that differs from said first force displacementcurve, and a third displacement curve associated with compression of thespine removed from said central zone that also differs from said firstdisplacement curve.
 33. A dynamic stabilization device, comprising: anelongate element defining a free end; a first resilient member mountedwith respect to the elongate element; a second resilient member mountedwith respect to the elongate element; a first attachment member mountedwith respect to the elongate element ; a second attachment membermounted with respect to the elongate member; and an abutment membermounted with respect to the free end of the elongate member; wherein thefirst resilient member is positioned between the first attachment memberand the second attachment member; wherein the second resilient member ispositioned between the second attachment member and the abutment member;wherein the first and second resilient members cooperate to deliver anon-linear force displacement response to relative movement between saidfirst and second attachment members; and wherein the first resilientmember and the second resilient member are subject to a compressivepreload.
 34. A dynamic stabilization device, comprising: an elongateelement defining a free end; a first resilient member mounted withrespect to the elongate element; a second resilient member mounted withrespect to the elongate element; a first attachment member mounted withrespect to the elongate element ; a second attachment member mountedwith respect to the elongate member; and an abutment member mounted withrespect to the free end of the elongate member; wherein the firstresilient member is positioned between the first attachment member andthe second attachment member; wherein the second resilient member ispositioned between the second attachment member and the abutment member;wherein the first and second resilient members cooperate to deliver anon-linear force displacement response to relative movement between saidfirst and second attachment members; and wherein the elongate memberpasses through an aperture associated with the second attachment member.